The cells involved in bone resorption and bone formation, i.e., osteoclasts, osteoblasts and osteocytes, will be studied in bone tissue maintained in organ culture as well as in animal models. Their activity will be assessed by quantitative ultrastructural morphology and these data will be correlated with biochemical data on the same tissue or in the same animal. We will test the effects of regulatory hormones on bone resorption in animals, such as vitamin D metabolites and the interaction of vitamin D with parathyroid hormone. We will test inhibitors of bone resorption in organ culture, such as calcitonin, diphosphonates and phosphates, and stilbamideine. We will study the effects of regulatory hormones on bone formation and on inhibition of bone formation in organ culture and test insulin, cortisol, somatomedin, thyroxine, vitamin D metabolites, parathyroid hormone, prostaglandins and osteoclast activating factor.